Milk proteins such as lactoferrin (LF), lysozyme (LZ), lactoperoxidase (LP), immunoglobulin-A (IgA), alpha-lactalbumin, beta-lactoglobulin, alpha-, beta- and kappa-caseins, serum albumin, lipase and others are known to have a number of nutritional and other beneficial effects, particularly for infants. Breast feeding of fresh human milk has traditionally been considered the best means to provide nutrition to an infant, especially during the first six months of life. Although all the physiological roles of human milk proteins have not yet been elucidated, evidence has been obtained that lysozyme, lactoferrin and other milk proteins can control the microflora in the gut of infants (Lönnerdal, 1985). Breast milk is a source of peptides, amino acids, and nitrogen and also contains casein and whey proteins, which are involved in the development of the immune response (e.g., immunoglobulins), together with other non-immunologic defense proteins (e.g., lactoferrin). Breast milk has additionally been suggested to contain many immune factors that compensate for the undeveloped defense mechanisms of the gut of infants (Saarinen K M et al., 2000). Several human milk proteins have been demonstrated to have beneficial physiologic effects in infants, particularly in the defense against infection and in the optimization of nutrient uptake.
However, many situations arise where the infant cannot be fed mother's milk and synthetic infant milk formulas are used in the place of breast feeding (Motil K J, 2000). Infant formulas and not standard cow's milk are used because (1) cow's milk has more than twice the protein of breast milk or infant formula and this protein may be hard for babies to digest; (2) the level of iron, zinc and vitamin C (which babies need in their diet) is low in cow's milk; and (3) the level of sodium level is three to four times that of breast milk and generally too high for infants less than a year old. A number of types of infant formulas which vary in caloric content, nutrient composition, digestibility, taste, and cost are available as an alternative to breast milk. Examples include standard cow milk-based formulas, soy protein formulas and formulas for premature infants or infants with special dietary needs due to allergies, etc. Considerable effort has been made to improve synthetic infant milk formulas in order to closely simulate mother's milk.
The protein and non-protein composition of the human milk and cow milk is described by Kunz et al., 1999. The relative concentrations of milk proteins vary between human and cows' milk. For example, lactoferrin and lysozyme are present in a relatively high amount in human milk but in only low or trace amounts in cow's milk.
In general, synthetic infant formula that has been prepared using cow's milk does not closely resemble the protein composition found in human milk. Accordingly, cow's milk based infant formula is typically supplemented with various human milk protein components. Typically, commercial infant formulas based on cow's milk contain approximately of 0.1 mg/mL lactoferrin whereas natural human breast milk contains an average concentration of 1.4 mg/mL. Soy-based infant formulas contain no added lactoferrin, lysozyme or other mammalian proteins.
Although addition of recombinant human milk proteins to infant milk or milk formula has been proposed, e.g., using transgenic cows or by addition of microbially produced human milk proteins to milk or milk formula, these approaches do not overcome the various problems of (i) allergies to cow's milk, (ii) the high cost of recombinant protein production and/or (iii) safety issues related to food products.
During the last several decades, improved infant formulas have become available that are safe and contain nutrient concentrations similar to, or higher than, breast milk. However, breast-fed infants still have a lower prevalence of infection than formula-fed infants and when they become ill, the duration of both diarrhea and upper respiratory infections is shorter than in formula-fed infants. (See, e.g., Kovar et al., 1984, and Dewey et al., 1995). In addition, it has been reported that breast-fed infants have a different growth pattern than formula-fed infants (Dewey et al., 1992; Dewey et al., 1993), and epidemiological studies show that they have a lower incidence of chronic diseases, such as diabetes and coronary heart disease.
It has been postulated that many of advantages to infants provided by mother's milk are effectuated through unique proteins present in breast milk, but not in baby formula (Lönnerdal, 1985). Human milk proteins are unique and even if the alternative protein sources used in infant formulas (e.g., skim milk, whey protein and soy isolates) mimic the amino acid concentration and ratio found in breast milk, the biological properties of human proteins cannot be readily copied.
Exemplary unique proteins present in human milk include lactoferrin and lysozyme. Lactoferrin is an iron-binding protein found in the granules of neutrophils which exerts an antimicrobial activity and lysozyme is a crystalline, basic protein present in saliva, tears, egg white, and many animal fluids, which functions as an antibacterial enzyme.
It is therefore desirable to provide transgenic plants that produce beneficial levels of proteins normally present in human milk, while largely avoiding costly recombinant protein production techniques and associated safety issues. More generally, it is desirable to provide a composition containing beneficial levels of proteins normally present in human milk that can be delivered by itself, as a nutraceutical or added to processed foods, for supplying one or more human milk proteins beneficial to human health.